TRANSFORMATION OF THE LARVA INTO THE ADULT. 153 



some of them, usually the second or third, and in many cases both 

 of these or even all the three anterior pairs temporarily degenerate, 

 the corresponding adult limbs growing out at the same points 

 (Dohrn, Hoek). In Pallene the second pair is wanting, and does 

 not even occur as a vestige, while in Tanystylum the first pair is 

 wanting as a functional appendage, but appears ontogenetically as a 

 well-developed pincer-carrying limb (Fig. 71 ^1 and B), and only 

 gradually degenerates in the later larval stages ; it is still present in 

 the adult as a small, vestigial two-jointed bud (Morgan). The 

 position of the second and third pairs of degenerated limbs is 

 marked by the appearance of the apertures of what are presumably 

 excretory glands (coxal glands). The tubular spine of the first 

 limb, through which the above gland opens, is thrown off in one of 

 the moults and gives place to an ordinary short spine. It has 

 therefore the significance of a larval organ. 



The first indication of the formation of new body-segments is, 

 according to Dohrn, found in a paired swelling of the intestine 

 behind the last of the larval limbs, accompanied by a bulging of the 

 body-wall. At the same time, in the posterior part of the ventral 

 surface, a thickening of the ectoderm appears which is the rudiment 

 of a new pair of ganglia. The ectoderm begins to become wrinkled 

 in the posterior part of the body and rises up above the newly- 

 formed lower layer. The larva now moults, after which it is evident 

 that a limb has appeared on the bulging of the body-wall just 

 mentioned ; into this limb an intestinal caecum is continued. It 

 is thus clear that this is a new limb, which soon develops and 

 becomes jointed (Fig. 71 ^4 and B). The other limbs form in the 

 same way. Only when the body thus lengthens do the three 

 anterior pairs of limbs also take part in the transformation (Dohrn). 

 The short abdomen arises as a posterior sac-like swelling, and the 

 anus appears upon it (Fig. 71 B). 



The transformation of the six-limbed larva just described takes 

 place in some forms, as has already been mentioned, within the 

 egg-envelope ; Nymphon brevicollum, for example, leaves the egg 

 when provided with five well-developed pairs of limbs (Fig. 72 

 A and B), and the first rudiments of a sixth pair. Other points 

 of its organisation, especially the shape of the limbs with the 

 intestinal caeca extending far into them, can be made nut without 

 further assistance from Figs. 72 A and B. The young of Nymphon 

 brevieaudatum possess all the limbs at hatching (Hoek), and the 

 same condition is found in the genus Pallene (Dohrn, Morgan). 



